The present application is based on Japanese Patent Application No. 10-172572 filed Jun. 19, 1998, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to molding of an ophthalmic lens product by using a mold assembly which has a mold cavity. In particular, the invention is concerned with the method of molding such a lens product having a configuration corresponding to that of the ophthalmic lens such as an intraocular lens or a contact lens, or a configuration corresponding to that of a precursor of the ophthalmic lens, by polymerizing a monomer composition in the mold cavity to provide a polymer product which constitutes the lens product.
2. Discussion of the Related Art
There have been known various methods for forming an ophthalmic lens product having a configuration or geometry corresponding to that of an ophthalmic lens such as an intraocular lens or a contact lens, or a configuration corresponding to that of a precursor of the ophthalmic lens, more precisely, a lens product which gives at least an optical portion of the intended ophthalmic lens. For instance, the lens product is obtained by effecting a machining operation on a bar-shaped or block-shaped polymer product, such as cutting and grinding. Alternatively, the lens product is obtained by a molding operation using a mold assembly which has a mold cavity. Namely, a monomer composition that gives the polymer product is introduced into the mold cavity of the mold assembly, and is polymerized to form an intended ophthalmic lens whose configuration corresponds to that of the mold cavity.
U.S. Pat. No. 4,113,224 and JP-A-4-290706 disclose one example of the method of molding the lens product by using a mold assembly consisting of a combination of two mold members such as a male mold and a female mold, a cavity-defining mold and a core mold, or an upper mold and a lower mold. The two mold members are assembled together so as to define therebetween a mold cavity having a configuration of the intended lens product. The mold cavity defined upon assembling of the two mold members is filled with a suitable monomer composition, and the monomer composition is polymerized to produce the lens product. In the mold assembly of the above US patents, a reservoir or storage chamber for storing an excess of the monomer composition is formed in a portion of the mold assembly radially outwards of the mold cavity. During the polymerization of the monomer composition in the mold cavity, the monomer composition accommodated in the reservoir or storage chamber is supplied to the mold cavity, so as to compensate for a volume of shrinkage of the monomer composition caused during its polymerization. Accordingly, the lens product obtained as a polymer product has a geometry following that of the mold cavity. In the method disclosed in the above U.S. patents, however, the polymerization of the monomer composition in the mold cavity is effected under the atmospheric pressure, and the monomer composition accommodated in the reservoir or storage chamber is not forced to flow into the mold cavity, but is fed thereinto by gravity. According to this arrangement, the monomer composition may not be supplied to the mold cavity in an amount sufficient to compensate for its polymerization shrinkage in the mold cavity. In this case, the molded surfaces of the lens product serving as the optical surfaces of the intended ophthalmic lens do not have the respective desired profiles which exactly follow those of the molding surfaces of the mold assembly defining the mold cavity. Thus, in the method disclosed in the above-indicated U.S. patents, the optical surfaces of the lens product do not have high accuracy of configuration.
U.S. Pat. No. 5,578,332 discloses a method of cast molding a contact lens by using a mold assembly consisting of a first and a second mold, which have respective molding surfaces for defining a mold cavity when they are assembled together. The mold cavity is filled with a monomer, and the monomer is polymerized to produce the intended contact lens with the mold assembly being subjected to superatmospheric pressure in a pressure vessel. The application of the superatmospheric pressure to the mold assembly accommodated in the pressure vessel ensures that the molding surfaces of the two molds are subjected to a loading pressure, so that they are deflected and kept in contact with the monomer as the monomer shrinks in volume during its polymerization. In this method, however, since the molding surfaces of the two molds are deflected due to the loading pressure acting thereon upon application of the superatmospheric pressure to the mold assembly in the pressure vessel, the profiles of the molding surfaces change so as to remain in contact with the shrinking lens monomer during its polymerization. This arrangement wherein the profiles of the molding surfaces change during the polymerization of the monomer may not provide the intended contact lens with high accuracy of configuration.
JP-A-9-28723 discloses a method of producing a soft intraocular lens by using a mold assembly consisting of an upper and a lower mold, which are assembled together such that they are fluid-tightly sealed to define therebetween a mold cavity which is filled with a monomer composition. The monomer composition in the mold cavity is polymerized with the mold assembly being subjected to a nitrogen pressure of 2.0 kg/cm2 at a temperature of 110xc2x0 C. for two hours. Since the mold assembly disclosed in this publication is fluid-tightly sealed, the monomer composition in the mold cavity is not subjected to the nitrogen pressure unless the molding surfaces deflect or deform so as to be in contact with the shrinking monomer composition in the mold cavity. In this case, the monomer composition in the mold cavity may not be formed into the intraocular lens having a desired configuration corresponding to that of the mold cavity. On the other hand, if the molding surfaces deflect or deform so as to be in contact with the monomer composition in the mold cavity which shrinks during the polymerization, the mold assembly may suffer from a problem similar to that experienced in the above U.S. Pat. No. 5,578,332. In other words, the intraocular lens to be obtained by using the mold assembly does not have optical surfaces with high accuracy of configuration due to the deflection or deformation of the molding surfaces.
It is therefore a first object of the present invention to provide a method of molding an ophthalmic lens product by using a mold assembly having a mold cavity and a monomer storage portion, which method permits production of the lens product whose molded surfaces corresponding to the optical surfaces of the intended ophthalmic lens have high accuracy of configuration, exactly following the profiles of the molding surfaces of the mold assembly, while compensating for a volume of shrinkage of the monomer composition during its polymerization, by effectively supplying the monomer composition from the monomer storage space into the mold cavity, so as to provide the lens product having a desired configuration.
A second object of the invention is to provide a mold assembly suitable for practicing the method of the invention.
The above first object of the present invention may be attained according to a principle of the invention, which provides a method of molding an ophthalmic lens product by using a mold assembly which has a mold cavity, the lens product having a configuration corresponding to that of an ophthalmic lens or a precursor of the ophthalmic lens and being obtained by polymerizing a liquid monomer composition in the mold cavity to provide a polymer product which gives the lens product, the method comprising the steps of: preparing the mold assembly having a monomer storage space for storing the liquid monomer composition, which storage space is held in fluid communication with the mold cavity; filling the mold cavity of the mold assembly with the liquid monomer composition while the liquid monomer composition is accommodated in the monomer storage space; and polymerizing the liquid monomer composition at a temperature of 10-80xc2x0 C. while a pressure higher than the atmospheric pressure is applied directly to a liquid surface of a mass of the liquid monomer composition accommodated in the monomer storage space.
In the above method of the present invention, the liquid monomer composition in the mold cavity is polymerized under application of a pressure higher than the atmospheric pressure directly to the liquid surface of the monomer composition accommodated in the monomer storage space which is held in fluid communication with the mold cavity. Owing to the pressure applied to the mass of the monomer composition in the monomer storage space in the mold assembly, the monomer composition present in the storage space is fed into the mold cavity via a passage formed in the mold assembly, to thereby effectively compensate for a volume of shrinkage of the monomer composition in the mold cavity during its polymerization. According to this arrangement, the optical surfaces of the lens product have a significantly enhanced accuracy of configuration without being adversely affected by the polymerization shrinkage of the monomer composition.
Although the monomer storage space in the mold assembly constructed as described above is open to the external space, the evaporation of the monomer composition is minimized by applying a suitable pressure higher than the atmospheric pressure to the monomer composition accommodated in the storage space. Further, the monomer composition is polymerized at a relatively low temperature in a range of 10-80xc2x0 C., to thereby effectively prevent the evaporation of the monomer composition, as compared when the monomer composition is polymerized at a relatively high temperature according to the conventional methods. Accordingly, the polymer product to be obtained does not suffer from a variation in its composition, in other words, the lens product to be obtained does not suffer from a variation of its properties, so that the lens product has an intended high quality.
In a first preferred form of the present invention, the mold assembly comprises: (a) a cylindrical body having an inner bore; (b) a lower mold half consisting of a cylindrical base portion which is fitted in a lower portion of the inner bore, so as to close a lower open end of the inner bore, and a cylindrical molding portion protruding from an upper surface of the base portion in an axially upward direction of the mold assembly and having an end face serving as a lower molding surface which has a profile following that of one of opposite surfaces of the lens product, the molding portion having a diameter smaller than that of the base portion so as to define a lower annular spacing between an outer circumferential surface of the molding portion and an inner circumferential surface of the cylindrical body; and (c) an upper mold half consisting of a cylindrical base portion which is fitted in an upper portion of the inner bore so as to partially define an upper chamber for accommodating the mass of the monomer composition, on the side of an upper open end of the inner bore, and a cylindrical molding portion protruding from a lower surface of the base portion in an axially downward direction of the mold assembly and having an end face serving as an upper molding surface which has a profile following that of the other surface of the lens product, the molding portion of the upper mold half having a diameter smaller than that of the base portion so as to define an upper annular spacing between an outer circumferential surface of the molding portion and the inner circumferential surface of the cylindrical body, the lower annular spacing and the upper annular spacing cooperating to provide an annular intermediate chamber, while the upper molding surface and the lower molding surface cooperating with each other to define therebetween the mold cavity such that a circumferential opening open to the annular intermediate chamber is formed along an entire circumference of the mold cavity, for introducing the monomer composition into the intermediate chamber, the base portion having a plurality of through-holes formed therethrough such that the through-holes are equiangularly spaced from each other in the circumferential direction of the base portion and communicate with the upper chamber and the intermediate chamber.
In the mold assembly constructed as described above, the mold cavity is filled with the monomer composition fed from the intermediate chamber via the plurality of through-holes formed through the upper base portion of the upper mold half, while the intermediate chamber and the mold cavity are deaerated via the through-holes. Thus, the through-holes of the present mold assembly serve as not only monomer inlet holes, but also deaerating holes. The present mold assembly does not require any light shielding means for protecting any portion of the monomer composition against exposure to a radiation used for the polymerization and any mechanism for removing the molded polymer product from the mold assembly, as provided in the conventional mold assembly, resulting in a simplified structure of the mold assembly. In the mold assembly used in the present invention, the plurality of through-holes are formed through the upper base portion of the upper mold half, such that the through-holes are equiangularly spaced from each other in the circumferential direction of the upper base portion, that is, in the circumferential direction of the mold cavity, to thereby permit a uniform or even flow of the monomer composition into the molding cavity via the through-holes. In addition, the intermediate chamber surrounding the mold cavity and filled with the monomer composition assures an effective flow of the monomer composition into the mold cavity, resulting in even or uniform supply of the monomer composition into the mold cavity to compensate for the shrinkage of the monomer composition during its polymerization. Accordingly, the lens product to be obtained has improved surface consistency.
Preferably, the mold assembly further comprises inlet defining means for defining an inlet at the upper open end of the inner bore, which inlet permits communication between the upper chamber and an exterior of the mold assembly, so that a pressurized gas is introduced into the upper chamber.
Preferably, in the present method, the step of filling the mold cavity comprises introducing the monomer composition into the intermediate chamber from the upper chamber, and feeding the monomer composition from the intermediate chamber into the mold cavity through the circumferential opening, so as to fill the mold cavity and the intermediate chamber with the monomer composition, and store the mass of the monomer composition in the upper chamber, and wherein the step of polymerizing the monomer composition comprises placing the mold assembly in a pressure vessel, and introducing the pressurized gas having a pressure higher than the atmospheric pressure into the pressure vessel, so that the pressurized gas acts directly on the mass of the monomer composition in the upper chamber through the inlet, and the monomer composition in the mold cavity, the intermediate chamber and the upper chamber is polymerized so as to provide an integral assembly which includes the polymer product and the lower and upper mold halves; and separating the lens product formed in the mold cavity from a rest of the integral assembly.
In a second preferred form of the present invention, the pressurized gas has a pressure which is higher than the atmospheric pressure by 0.05-1.0 MPa.
In a third preferred form of the present invention, the step of polymerizing the monomer composition is effected at a temperature in a range of 30-60xc2x0 C.
The above-described second object may be attained according to the present invention.